JP3042720B2 - Deterministic magnetorheological finishing equipment - Google Patents

Abstract

A method and apparatus for finishing a workpiece surface using MR fluid is provided wherein the workpiece is positioned near a carrier surface such that a converging gap is defined between a portion of the workpiece surface and the carrier surface; a magnetic field is applied substantially at said gap; a flow of stiffened MR fluid is introduced into said converging gap such that a work zone is created in the MR fluid to form a sub-aperture transient finishing tool for engaging and causing material removal at the portion of the workpiece surface; and the workpiece or the work zone is moved relative to the other to expose different portions of the workpiece surface to the work zone for predetermined time periods to selectively finish said portions of said workpiece surface to predetermined degrees.

Description

Description: TECHNICAL FIELD The present invention relates to a method and apparatus for finishing using a magnet-rheological fluid and to the composition of the fluid used therein.

Background Art [0002] Processes for finishing workpieces such as optical lenses generally involve removing material on the surface of the workpiece for three purposes:
(1) removal of surface damage, (2) smoothing of the surface, and (3) achievement of shape correction. Many well-known polishing processes can achieve the objectives (1) and (2), but it is difficult to achieve the objective (3). Examples of such processes include full aperture contact polish on pitch wrap or polyurethane wrap. Such a process is generally inefficient in correcting the shape of the optical lens and often requires a lot of intermediate work. Other technologies include ion beam
Milling can achieve objective (3), but is not efficient with respect to objectives (1) and (2). Ion beam milling cannot be smoothed and has been demonstrated to cause subsurface damage if not precisely controlled.

The finishing of precision optic lenses typically requires that the surface conforming to the desired shape be produced with a height range of 0.50 microns. Optical lens finishing typically requires removal of material at a relatively high rate, even if it is a material such as hard glass. The optical lens finish also removes subsurface damage from previous grinding operations to 20Årms
Alternatively, removal of sufficient material to achieve microroughness less than or equal to this is a typical requirement.

Conventional finishing processes use precisely shaped viscoelastic pitch or polyurethane foam wrap to transfer pressure and velocity through the abrasive slurry to the workpiece. This wrap is called a full aperture wrap because it is large enough to cover the entire optically practical portion of the lens. The work surface of the finishing tool must match the surface of the desired work piece. If the viscoelastic finishing tool is flexible, as in the case of a tool made of pitch, rosin or wax, it will deform under the influence of the pressure and heat generated during the finishing process to deform to the desired shape. Lost and uncorrected work piece surface shape. While such tools can continue to smooth the surface, their ability to correct the surface shape is significantly reduced. The finishing tool must be reshaped with a metal template that is machined to the desired shape before starting the finishing again. Such an intermediate step is unexpected and time consuming. This requires a high level of technician or specialized optical technician. This also requires an inventory of metal templates for each workpiece shape.

Also, visco-elastic finishing tools can be less flexible, as in the case of tools made from rigid thin polyurethane pads mounted on a metal backing template. While this type of finishing tool is better at maintaining the desired shape during the finishing process,
This wears out over time and reduces the removal rate. It is difficult to achieve the required level of surface smoothness because such tools have a reduced ability to smooth the work piece surface. A professional optical technician must periodically stop the process and clean or replace the pads to continue the finishing process.

Conventional full aperture, viscoelastic finishing tools all suffer from blockage of particulate matter. Broken glass and / or
Alternatively, abrasive polishing particles may clog the surface of the tool over time. This surface becomes glassy and smooth.
This reduces the removal rate. Also, clogged particulate matter can damage the surface of the work piece and damage the work piece in the final finishing stage. Such tool degradation is unpredictable. The reason is that finishing a complicated surface is a troublesome operation and mass production is difficult.

Some finishing operations use a sub-aperture wrap, ie, a finishing tool that is smaller than the portion of the workpiece that needs to be finished. Regarding this, for example, U.S. Pat.
See 956944 (Ando et al). However,
Since such a process uses a solid finishing tool, it suffers as many disadvantages as a process using a full size wrap.

Milling processes, including using a solid tool and using ion beam bombardment, can also use sub-aperture wrap. Such a process can impart shape to the work piece, but cannot smooth the surface and roughens the surface by exposing subsurface damage.

It is known to use fluids containing magnetic particles in polishing applications. U.S. Patent No.4821466
(Kato et al) disclose a polishing process in which a "float pad" immersed in a fluid containing colloidal magnetic particles is pressed against a workpiece by buoyancy generated by applying an unequal magnetic field. This polishing process has the same basic shape correction capabilities as full aperture, viscoelastic finishing tools are used. The shape of the float and the shape of the magnetic field must be tailored to obtain a particular desired surface shape. Finishing different shapes in the same process requires different float designs and fabrications with different lapping motions and possibly different magnetic shapes. In order to change the shape of the optical lens in this way, substantial processes and machine modifications are required.

It is also known that a work piece is polished by immersing the work piece in a fluid containing magnetic particles and applying a rotating magnetic field to the fluid. For example, U.S. Pat. No. 2,735,232 (Simj
ian). It is said that this rotating magnetic field causes the fluid to flow in a circular shape around the work piece to polish it. This method has the disadvantage that a satisfactory material removal rate cannot be obtained because sufficient pressure is not generated on the workpiece. In addition, this method cannot correct a surface shape error to such an extent that optical requirements are satisfied.

DISCLOSURE OF THE INVENTION In view of the above description, it is an object of the present invention to provide an improved finishing method and apparatus using a magnet rheological fluid. It is a further object of the present invention to provide a magnet rheological fluid for use in the method and apparatus.

It is a further object of the present invention to provide a finishing device that can be used in finishing optical lenses.

It is a further object of the present invention to provide a high degree of smoothness by removing substantially existing surface and subsurface damage without causing substantial surface or subsurface damage (scratch, crack or subsurface crack). The purpose of this is to provide a finishing device.

It is a further object of the present invention to provide a finishing device that provides surface smoothness and shape correction.

It is a further object of the present invention to provide a finishing device that can be easily automated and that is flexible in its application.

It is a further object of the present invention to provide a means for automating this device.

It is a further object of the present invention to provide a finishing device that operates at a relatively high removal rate for various materials.

It is a further object of the present invention to provide a finishing apparatus for smoothing the surface of a wide variety of materials according to accepted precision optical standards.

It is a further object of the present invention to add a nanocrystal diamond abrasive to a reference MR fluid composition or other utilized MR fluid composition for hard materials such as silicon or particularly hard materials such as sapphire for removal rates. It is an object of the present invention to provide a finishing apparatus capable of accelerating the finishing.

It is a further object of the present invention to automatically adjust to any workpiece surface shape, whether the finishing tool is convex, concave or flat, in which case the configuration of the finishing machine, such as the exchange of precision shaped laps. The goal is to provide a finishing device that does not require modification.

It is a further object of the present invention to provide a finishing device in which the finishing tool has a function of removing the shape of the finishing spot.

It is a further object of the present invention to provide a finishing device that includes a magnet rheological fluid that is disproportionate in operating conditions of heat, polishing and air exposure.

These and other objects are achieved by an MR finishing method and apparatus according to the present invention. This method of finishing the surface of a work piece using MR fluid involves positioning the work piece near a carrier surface so that a narrowing gap is formed between a portion of the work piece surface and the carrier surface. Substantially applying a magnetic field to the gap; introducing a flow of a magnetically hardened MR fluid into the narrowing gap to form a machining zone with the MR fluid; Forming a mobile finishing tool that removes material in combination with the workpiece or the processing zone relative to the other to move different portions of the surface of the workpiece relative to the processing zone. Selectively exposing the portion of the work piece surface to a predetermined extent by exposing for a predetermined time. An apparatus that finishes a work piece using MR fluid is composed of a carrier surface that conveys MR fluid, and a part of the surface of the work piece that holds the work piece and is positioned near the carrier surface and between them. A piece holder that forms a narrowing gap and is processed so that the carrier surface conveys an MR fluid passing through the gap; and an MR that flows through the gap by applying a magnetic field to the gap.
A magnet forming a moving finishing tool that hardens fluid and engages a portion of the surface of the workpiece to remove material; and a workpiece that moves the workpiece or the machining zone relative to the other. Means for exposing different portions of the surface of the work piece to the working zone for a predetermined time to selectively finish the surface portion of the work piece to a predetermined degree.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a finishing apparatus as an example of the present invention, FIG. 2 is an enlarged view of a part of the apparatus of FIG. 1, and FIGS. 3A to 3C are used for finishing convex, flat and concave workpieces. FIG. 3D is a cross-sectional view of FIG. 3C with a pole piece added, and FIGS. 4A and 4B are schematic illustrations of a carrier wheel and MR fluid ribbon using a circular and flat carrier surface. FIG. 5A is a cross-sectional view of an example pole piece according to the present invention, FIG. 5B is a front view of the pole piece, FIG. 5C is a plan view of the pole piece, FIG. 6A and FIG. 6B are the present invention. Field plots of the magnitude and direction of the magnetic field in and surrounding the pole piece, based on FIG. 7, FIG. 7 is a schematic diagram of a fluid circulator as an example of the present invention, and FIG. 8A is in contact with a vertical wheel. FIG. 8B is a cross-sectional side view of a fluid supply nozzle of the present invention. 9A is a perspective view of a workpiece that is shaped using the scraper of FIG. 9B, FIG. 9B is a perspective view of a scraper used to form a serrated pattern in MR fluid, FIG. 9B is a profile of the work piece of FIG. 9B taken with a Rank Taylor Hobsen From Talysurf profiler, FIG. 9D is a perspective view of a scraper used to form a triangular MR fluid ribbon, and FIG. 10A is a fluid collector as an example of the present invention. FIG. 10B is a front sectional view of the collector, FIG. 10C is a bottom view of the collector, FIG. 11A is a schematic view of the reservoir of the present invention, FIG. 11B is a schematic view of another reservoir used in the present invention, 12 is a graph showing the finish spot width and length obtained with varying sized abrasive grit; FIG. 13 is a graph of 6 for three MR fluids made with different carrier fluids, normalized to the starting speed of one unit. Time or more Graph showing the volumetric removal rate measured over, Figure 14A MRF removal function shown in FIGS. 14B and 14C "spot"
14B and 14C show the M of BK7 glass after a 5 second finishing operation.
Figures 15A-15C show the results of finishing spots on spinning workpieces, Figure 16 shows the flow chart of the computer controlled algorithm of the present invention, Figures 17A and 17B show the results on fused silica. FIG. 18 shows the MRF removal function “spot”, FIG. 18 is a portion of the software user interface used in an embodiment of the present invention, showing the initial, expected, FIGS. 19A-19C are schematic diagrams of alternative finishing devices possible with the present invention, and FIGS. 20A and 20B are schematic perspective views of alternative finishing devices possible with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Magnet-rheological ("MR") fluids are fluids composed of non-colloidal magnetic materials evenly dispersed in a carrier fluid, which, when placed under a magnetic field, have rheological properties ( (Plasticity, elasticity and apparent viscosity) or other fluid properties. Typical applications of known magnetic fluid compositions have been as shock absorbers, clutches and actuating modules.

The present invention relates to an improved method and apparatus for magnet rheological finishing of a work piece surface. In the present invention, the surface of the work piece is placed on the carrier surface with a gap therebetween.
The MR fluid is placed on a carrier surface, which then carries the MR fluid through the gap. A magnetic field is applied to the gap to substantially harden the MR fluid flowing through the gap to form a temporary processing zone or finish spot to remove material on the workpiece surface. The finishing spot is smaller than the work piece surface, and the work piece can move on the finishing spot by moving the work piece relative to the carrier surface. By controlling the dwell time of the spot at different locations, the process can achieve surface smoothness, removal of subsurface damage or shape correction with the desired high tolerance. In this workpiece finishing step, the MR fluid carries away heat, abrasive particles and particles of the workpiece material and continuously supplies fresh abrasive particles to the processing zone.

FIG. 1 shows an MR finishing apparatus 10 according to the present invention as an embodiment.
This device 10 includes a vertical carrier wheel 12,
The wheel includes an outer rim 14 that forms a carrier surface (shown in more detail in FIG. 2). (Although not shown, the carrier surface may include the inner bottom wall of a circular trough, the top surface of a rotating belt or other suitable moving surface. Other possible shapes are disclosed in FIGS. 19 and 20. This vertical wheel supports a ribbon-shaped volume of MR fluid 16. The MR fluid ribbon is placed on one side of the wheel by the fluid supply nozzle 18 and is conveyed to a remote side where it is collected by the fluid collector 20 by rotation of the wheel. The wheel transports the ribbon through a gap 22 between the workpiece surface and the carrier surface where the magnet provides the optimum magnetic field. The vertical carrier wheel is preferably formed of a non-magnetic material such as aluminum or plastic.

Preferably, the carrier wheel is cylindrical rather than flat and has a convex curve over its width.
In a preferred embodiment, the wheel has a spherical cross section. That is, the radius of curvature across the width of the rim is equal to the radius of the wheel on its circumference. The advantage of a carrier wheel whose finishing spot is a convex curved surface is that it can be used for finishing flat surfaces and concave surfaces as well as convex surfaces as shown in FIG. As a result, the carrier wheel of the present invention can be used for finishing any shape in its vertical direction, for example, a torus shape and a cylinder. However, wheels dedicated to flat surface finishing advantageously have a flat rim across their width.

Another advantage of using a vertical carrier wheel is that it makes the MR fluid contact the workpiece at a steeper angle than a flat wheel. As shown in FIG. 4, the flat carrier surface 24 is formed by applying an MR fluid ribbon to a workpiece 26.
In contrast to this, the carrier wheel 12 strikes the MR fluid ribbon against the workpiece 26 at a relatively obtuse angle, as opposed to typically at an angle close to the tangent. As a result, the ribbon is less likely to be disturbed by the outer edge 28 of the workpiece when it comes to the finishing spot. This advantage can be obtained without reducing the removal rate by using a vertical carrier wheel.

The magnetic field at the gap can be created by any means, including electromagnets and permanent magnets. In a device according to an embodiment of the invention, the magnetic field is formed by a DC electromagnet, which comprises a pole piece located below the carrier surface for applying a magnetic field to the MR fluid. The amount of space directly corresponding between the pole pieces can be referred to as the magnetic gap. The magnetic field outside this magnetic gap can be referred to as the outer piece magnetic field. The magnetic field line in the outer piece magnetic field is an arc connecting both poles. The magnetic gap between the pole pieces of the present invention is located below the carrier plane. The pole piece of the present invention can be used with the vertical wheel of FIG. 1, or any other suitably shaped carrier surface. This MR fluid ribbon is transported by the carrier surface through an outer magnetic field. If the carrier surface is the rim of a vertical wheel, the pole pieces are located below the rim on either side of the wheel.

The present invention contemplates pole pieces that form an optimal outer piece field where a finishing spot or working zone is formed in the gap between the work piece and the carrier surface. The pole pieces are also preferably set to minimize magnetic field strength at the fluid collector to prevent hardening of the MR fluid and facilitate fluid pick-up. Both of these objectives can be achieved by pole pieces that create a high marginal magnetic field above the magnetic gap, i.e. in the direction of the finishing spot, and a low marginal magnetic field below the magnetic gap.

An example of a pole piece design that is expected to have this feature when used in a vertical carrier wheel is shown in FIG. 5 as an example. FIGS. 6A and 6B are cross-sectional profiles of a pole piece for this embodiment, including the field magnitude inside and within the pole piece and surrounding magnetic field magnitude and direction. This design also has the advantage that it can be formed by conventional processing techniques rather than CNC processing, which can substantially reduce manufacturing costs.

This exemplary device includes a fluid circulation system (FIG. 7). This fluid circulation system includes a fluid supply nozzle 18,
It includes a fluid collector 20, and a device for recirculating fluid from the collector to a supply nozzle. The fluid circulation system of the present invention can be used on any carrier surface, including the carrier wheel shown in FIG.

In order to reduce evaporation of the carrier fluid due to air contact and deterioration of the MR fluid, this fluid circulation system should not expose the MR fluid to the atmosphere except for a ribbon of fluid temporarily present on the carrier surface. preferable. Since this fluid circulation system is external to this finishing device,
The MR fluid can be subject to any number of control functions between the collector and the nozzle, increasing the productivity and predictability of the MR fluid ribbon.

The fluid supply nozzle 18 used in the vertical carrier wheel 12 is shown by way of example in FIG. The fluid supply nozzle is preferably made of a material having a low magnetic energy, such as iron. This magnetically low energy nozzle
Isolates the MR fluid from the magnetic field and prevents it from hardening before leaving the nozzle. The nozzle and supply tube preferably provide a laminar flow of MR fluid. This nozzle may have a tapered interior. The nozzle may have its discharge end in direct contact with the carrier surface,
Alternatively, it may not be direct. For direct contact, applying a Teflon or similar coating to the carrier surface is advantageous in preventing wear. Preferably, it is tangential to the orbit carrier surface of the fluid exiting the nozzle.

In one embodiment, the MR fluid ribbon is formed by a fluid supply nozzle when the MR fluid is placed on a carrier surface. In another embodiment, the ribbon is formed by a scraper in contact or near contact with the carrier surface. In this case, the scraper has an opening that forms an MR fluid that is placed on the carrier surface by the nozzle. In this embodiment, since the MR fluid forms a pool behind the scraper, a circular trough-like side wall (not shown) can be provided on the carrier surface. The nozzle or scraper is preferably located in an area that is exposed to some magnetic field strength so that the MR ribbon has the flexibility to substantially maintain the shape it receives from the nozzle or scraper.

The shape of the nozzle or scraper is one factor that determines the cross-sectional size and shape of the ribbon. This also affects the size of the finishing spot. That is, narrow ribbons form narrow finish spots. Narrow spots can provide higher resolution in the finishing process and are therefore particularly useful for finishing very small workpieces. FIG. 9D is a perspective view of the scraper 48, which is used to form a triangular ribbon with a triangular cross section. This "tapered" ribbon was used to perfect a lens with a diameter of 5mm.

FIG. 9B is a perspective view of the scraper 50, which is used to form a sawtooth ribbon of this tooth pattern.
The scraper was created to show that it can maintain this shape and form a ribbon that moves this shape to the work piece. FIG. 9A is a perspective view of an originally flat work piece made of K7 glass, formed by contacting the ribbon formed by the scraper of FIG. 9B for 5 minutes. Figure 9C shows Rank Taylor Hobs
Figure 9A taken with the en From Talysurf® profiler
The profile of the work piece.

FIG. 7 is a schematic diagram of a fluid circulation system. As shown, the MR fluid can be pressurized with one or more feed pumps. The fluid circulation system supplies the MR fluid to the carrier surface at a linear velocity equal to or higher than the linear velocity of the carrier surface. When the MR fluid supply speed is lower than the carrier surface speed, a discontinuous ribbon is formed. When the fluid supply speed is faster than the carrier surface speed, a thicker ribbon is formed. Ribbon thickness can be controlled by changing the MR fluid supply rate. Expressed mathematically, the fluid supply rate Q (cm ³ / sec) is equal to the ribbon cross-sectional area S (cm ² ) times the linear velocity V (cm / sec) of the carrier surface. That is, Q = S × V. As described above, for an arbitrary carrier surface speed, an increase in the fluid supply speed Q results in an increase in the ribbon cross-sectional area S. Similarly, any fluid supply speed Q
The reduction in the carrier surface velocity V is caused by the ribbon cross-sectional area S
Increase.

Fluid collector 20 (FIG. 10) includes a pickup scraper 52, such as rubber, flexible plastic, etc., that functions as a vacuum cleaner to separate fluid from the carrier surface. The wheel engaging portion of the pickup scraper should conform to the shape of the carrier surface. The wheel engaging portion is preferably cup-shaped or U-shaped, into which the MR fluid ribbon enters from the opening side. The fluid collector 20 is used to suck MR fluid.
Preferably, it is connected to one or more suction pumps. The fluid collector advantageously includes or is covered by a magnetic shield of a magnetically low energy material such as iron. The magnetic shield substantially frees the MR fluid from the effects of the surrounding magnetic field, allowing the fluid to return to a less viscous state. Furthermore, because this collector is farther from the pole piece than the nozzle,
This is advantageous for reducing the degree of exposure to a magnetic field.

Since this circulation system uses a peristaltic pump, it has the advantage of reducing the number of contacts between the MR fluid containing abrasive particles and easily deteriorated components that are difficult to exchange. In peristaltic pumps, only the part exposed to wear by the MR fluid is a short tube, which can last for hundreds of hours and can be replaced cheaply. Peristaltic pumps are themselves relatively inexpensive. The pump was found to operate at low flow speeds without gaps in the ribbon. Two or more pumps can be used in parallel to adjust the pulse generation to reduce the amplitude. In the preferred embodiment, two three-feed pumps 32 are used, the drive heads of which are offset by 60 ° with respect to each other.

MSTERFLEX® 6485-82 PharMed® tubing can be used as the suction for this fluid circulation system. IMPERIAL-EASTMAN 3/8 tubing can be used as a supply for this fluid circulation system. EASY LOAD MASTERFLEX. (Registered trademark) pump, m
od. no. 7929-00 can be used as the feed pump. COLE-PALMER MASTERFLEX (registered trademark) pump, mo
d.no. 7019-25 can be used as a suction pump. Permanent magnet motor mod.no.2M168C, Dayton can be used to drive the pump with DC speed control mod.no.5X485C, Dayton.

Removed from carrier wheel by collector
The MR fluid can be sent to a reservoir 36 as shown in FIG. 11A. PP NALGENE®, a 1000 ml separation funnel can be used as a reservoir. The MR fluid is preferably supplied to such a reservoir with sufficient force to homogenize the MR fluid by disrupting the remanent magnetic particle structure formed by the applied magnetic field. However, the reservoir can also include a rocking device such as a stirrer 38 for this purpose. Laboratory stirrer TLINE, mod.no.102, can be used for this purpose. Also, other stirring or homogenizing devices can be used. The reservoir may be of a non-magnetic, wear-resistant material such as stainless steel. This can be conical or other shapes that do not provide a sedimentation zone where the MR fluid can wander. This allows the shape of the stirrer to fit over a large volume reservoir and leave no sedimentation zone (FIG. 11B).

The fluid circulation system can also include a temperature controller, such as a cooling mechanism, to remove heat generated in the finishing zone and carried away by the MR fluid. The temperature of MR fluid is
It is also increased by the operation of the MR fluid circulation pump or the heat generated by the electromagnet. The viscosity of the uncontrolled high heat MR fluid is reduced to increase the evaporation rate of the carrier fluid. Uncontrolled high heat can also cause thermal expansion of the product of the device, resulting in inaccurate positioning of the workpiece in the MR fluid, resulting in loss of shape control. In an exemplary device, the MR fluid is cooled by immersing a cooling coil 40 in a reservoir. Constant temperature cooling water is supplied to a cooling coil connected to a closed loop cooler such as Brinkman Lauda RM6. MR
The temperature of the fluid is typically maintained at about 21-22C.

The fluid circulation system may also include a composition controller, such as an automatic viscosity control system, to recover the loss of the carrier fluid from evaporation or other causes from the MR fluid. The automatic viscosity control system 54 can maintain the MR fluid at a constant viscosity by automatically dripping the carrier fluid into the reservoir to compensate for the loss. The viscosity control system may include a viscosity monitor operatively connected to the carrier fluid pump 44 with a reservoir of the carrier fluid 56. In the exemplary apparatus shown in FIG. 7, the viscosity is monitored using one or more pressure probes in the feed line. This is because the change in pressure between the supply pump and the supply nozzle is proportional to the change in viscosity for a constant flow rate. Preferred pressure probes
A diaphragm sensor like the Cooper PFD 102.
This reduces the stagnation point as a cause of MR fluid settling in the line and clogging the sensor. This pressure probe signal (or the difference between the signals from successive pressure probes if multiple probes are used) is proportional to the viscosity of the MR fluid if the fluid flow is constant. The pressure probe signal (or difference signal) is compared to a reference value, and if the signal exceeds the reference value, an error signal is sent to an electrical relay or motor driver to cause the carrier fluid mini-pump 44 to send the signal to the reference value. It works until it becomes below. In some embodiments, a proportionality constant between the pressure probe signal (or difference signal) and the error signal must be selected to avoid over-compensation (occurrence of vibration) or under-compensation (slow viscosity control).

Also, the concentration of the magnetic particles in the MR fluid can be monitored using an inductance probe, for example, a wire coil wound around a tube carrying the MR fluid. High inductance readings from this coil indicate high magnetic particle concentration and high MR fluid viscosity. However, this technique does not detect changes in viscosity that are problematic due to temperature changes or concentration of non-magnetic particles. This should be considered as a secondary indicator of the stability of the MR fluid. Pressure measurements proved to be more sensitive than inductance measurements.

The selection of a stable MR fluid significantly increases the reproducible and predictable finishing efficiency. The composition of many MR fluids is well known in the art, including oil-based and water-based fluids. This application is preferably based on aqueous carrier fluids for most applications
Consider the use of MR fluid. However, aqueous carrier fluids are not suitable for use on water-soluble workpieces, such as workpieces made of KDP (KH ₂ PO ₄ ) crystals.

This MR fluid contains non-colloidal magnetic particles, such as carboxylic iron particles. Table 1 shows four carbonyl iron powders obtained from GAF which were found to be effective in MR fluids.

To increase the ability to remove material, the MR fluid also includes a non-magnetic polishing material such as cerium oxide (CeO ₂ ). The choice of non-magnetic abrasive material is based on the physical properties (ie, hardness) and chemical properties (ie, chemical durability) of the workpiece to be finished. Table 2 is a list of recipes for MR fluids, which were used to finish fused quartz workpieces with the method of the present invention using equipment using a rotating trough carrier surface.
5 is a list using various abrasive particles and removal rates obtained when using a fluid. The removal rate was measured by comparing the profile of the work piece before and after taking it with a Zyogo Mark IV xp® interferometer. The first two formulations contain no additional abrasive. These are based on the polishing properties of carboxylic iron alone. Table 3 is a list of removal rates for various workpiece materials,
A standard MR fluid formulation containing a cerium oxide abrasive (Formulation D in Table 2) and a cerium oxide and nano diamond abrasive (Table 2) using the method of the present invention in an apparatus using a rotating trough carrier surface. Fig. 2 is a list obtained with advanced formulas including formula E). This data shows that the method of the present invention is effective in finishing very hard materials such as sapphire (AI ₂ O ₃ ).

It is an advantage of the present invention that the finishing spot is relatively insensitive to the size of the abrasive particles. FIG. 12 is a graph showing the width and length of the finishing spot obtained by changing the size of the abrasive particles. This finishing spot is Zygo Mark IV ex
(Registered trademark) measured with an interferometer. The spot size is relatively constant between 2 and 40 microns of particles. A further point of the invention is that unwanted oversized abrasive particles are relatively trouble free. This is because, unlike solid wrap, these particles do not clog and damage the surface of the work piece.

The MR fluid also contains a stabilizer such as glycerin.
The stabilizer is used to add viscosity to the MR fluid to create a situation that keeps the magnetic and abrasive particles suspended. However, excessive use of stabilizers such as glycerin is detrimental to finishing materials such as silicate glasses. Glycerin is thought to interfere with the action of water to hydrate and soften the surface of the glass.

Any form of degradation of this MR fluid can cause problems in MR finishing. This is because unstable MR fluids do not produce as many predictable finish spots. Rust can cause stability problems in MR fluids of the type of the present invention. This fluid uses finely crushed iron particles as an aqueous slurry. Because iron oxide has different magnetic properties than carboxyl iron, the magnetic properties of rusted MR fluids are constantly changing, and thus rust causes unpredictability. In addition, rust can stain the workpiece.

Since the MR fluid is partially exposed to the atmosphere, it absorbs carbon dioxide and lowers the pH of the fluid, promoting metal oxidation. Although the use of deionized water can slow down corrosion, it does not completely solve the problem,
Inconvenience and expense will increase.

It has been found that the addition of an alkali sufficient to raise the pH to about 10 improves the stability and the removal rate. Particularly effective alkalis in this application are buffers such as Na ₂ CO ₃ . Further, the advantage of using an alkaline buffer is that there is no need to use deionized water, and tap water can be used instead. FIG. 13 shows the volume removal rate normalized to the starting rate of one unit, showing three different carrier fluids:
Deionized (DI) water at pH 7, DI water adjusted to pH 10 with NaOH, and N
Although it was measured over a 6 hour or more time periods with respect to MR fluids made with tap water and pH10 with a ₂ CO ₃ shows a graph of the velocity. (Note that this DI water is theoretically assumed to have a pH of 7, which is ion free and its pH cannot be measured using conventional probes.) In a device using a rotating trough carrier surface, the same prescription is used, except for the different carrier fluids, using the same workpiece, according to the method of the present invention, the removal rate of which is measured as described above. Was. Na ₂ C
The removal rate of the pH 10 fluid containing O ₃ was high, but the removal rate of the pH 10 fluid containing NaOH dropped to 80% of the initial rate after 7 hours of use, and the removal rate of the pH 7 fluid after 2 hours. Became unstable by dropping to about 60% of the initial value. Table 4 shows that the volume removal rate (volume of material removed per second) increased by about 30% and the peak removal rate (depth of material removed per second).
Is increased by about 50%, as compared to the carrier fluid at pH 7 (Steps 3 and 4) using the pH 10 carrier fluid (Steps 1 and 2). The finishing steps in Table 4 were performed on the same work piece in an apparatus using a rotating trough carrier surface, and the removal rates were measured as described above.

The present invention contemplates MR fluids. The MR fluid consists MizuNaru carrier fluid, the carrier fluid comprises an alkali buffering agent such as Na ₂ CO _3, which demonstrates improved stability, the removal rate is resistance and improvements to rust, also Can be prescribed with tap water.

The workpiece 26 to be finished is a rotatable workpiece spindle
It is mounted on a workpiece holder consisting of 46 pieces. This support is preferably a non-magnetic material. The spindle is lowered and the workpiece is brought into contact with the MR fluid ribbon to form a finishing spot (FIG. 14). Because the angle θ can be changed, the finishing spot moves from the center of the lens to the edge (Fig.
15). Since the workpiece is rotationally symmetric, this spindle rotates the workpiece about the spindle axis. Since the workpiece is rotating and moving, the finishing spot removes material from the center to the edge of the workpiece in a ring. The pause time at each position on the lens is controlled so that the shape error of the workpiece is corrected. The determination of the pause time and the control of the movement of the spindle are performed by a computer.

Is measured relative to the vertical axis. This spindle turns around a pivot point within a range of an angle θ. This spindle can be turned in any direction within the range of the angle θ, but is preferably turned in a direction parallel to or perpendicular to the direction of motion of the MR fluid. In the apparatus of FIG.
The wheel 12 can rotate about the Z axis, and the angle θ
Allows the operator to change the direction of rotation relative to the direction of motion of the MR fluid.

Normally, the rotational speed of the spindle is kept constant.
Typical speed is 75 rpm. However, the spindle speed can be varied as a function of the rotational position of the spindle to finish the rotationally non-symmetric workpiece or to compensate for the rotationally non-symmetrical flaws. For a workpiece such as a cylinder, the movement of the spindle can be a moving and pivoting movement without rotating it. For a flat work piece, the movement of the spindle can be a combination of movement and rotation without rotating it, or the movement can be performed in a raster pattern.

In some systems in which the finishing spot is moved over the workpiece surface, the angle θ is only variable. In this system, the spindle is lowered until the workpiece contacts the MR fluid ribbon. The spindle is then angled about a mechanical pivot point, i.e., the B axis, which consists of a rotating joint that holds the spindle above the carrier plane.
Is rotated in the range. This B axis is parallel to the Y axis as shown in FIG. In this system, a constant processing gap (i.e., the gap between the workpiece and the carrier surface) is maintained if the workpiece is spherical, but not if the workpiece is spherical. However, it is an advantage of the present invention that the finishing spot allows a large change in gap height. For this reason, spherical workpieces can be finished if the spindle is limited to spherical motion, as shown in the example in FIG.

Another system for rotationally polishing a symmetric workpiece is shown in FIG. In this embodiment, the movement of the spindle is freely decelerated in three steps separately from the movement of rotating the workpiece. The movement of the spindle axis is limited to the XZ plane. This spindle moves up and down along the Z axis,
It moves left and right along the axis and rotates clockwise or counterclockwise about the B axis within an angle θ.

This device also has two passive degrees of freedom. The carrier wheel and its supporting base are manually rotated about the Z axis such that the wheel is parallel or perpendicular to the X axis. The spindle is manually moved along the Y axis to allow for fine adjustment of the spindle and wheels during assembly of the device.

The work piece can be operated by synchronously moving these action axes, the machining gap is kept constant,
The finishing zone is moved along the diameter from the center of the workpiece to its edge.

Control of the movement of the spindle arm can be effected by suitable mechanical means. The operation of the spindle arm controller can be controlled by a computer.

Computer controlled finishing of the parts can be accomplished with the steps shown in FIG. Forbes-Dums Finishing Algorithm
Computer code called (FDFA) is used. This requires three inputs. A) the shape and size of the MRF removal function or finish "spot"; B) the shape of the initial work piece surface; and C) the subject of processing. For example,
dc material removal, shape correction, or both. As output, this FDFA generates a device control operating program known as MCOP. The FDFA also gives notice of residual surface shape errors remaining in the processed member. This MRF device is an MCO
Finishing of the work piece is controlled by P.

This MRF removal function can be obtained by generating a spot of a test piece having the same material and shape as the object to be finished. The interferogram of this ablated “spot” is recorded by an interferometer such as the Zyogo Mark IV ex®, read out, and
Loaded on cord. Alternatively, a pre-recorded and stored "spot" profile can be read from the database and used.

This finishing spot is specific to the device platform, magnetic field strength, workpiece profile, carrier surface speed, MR fluid properties, spindle / carrier surface profile, and the properties of the material being finished. FIG. 14B shows the removal “spot” (the direction of fluid movement indicated by the arrow), in which a BK glass lens with a diameter of 40 mm and a radius of curvature of 84 mm contacts the MR fluid at a height of 1 mm above the carrier surface for 5 seconds. It shows the results obtained. An apparatus using a rotating trough carrier surface was used. For this device, the radius from the center of the trough to the inner edge is 23 cm and the radius from the outer edge to 30
cm. The trough was rotated at 20 rpm and the strength of the magnetic field in the gap was 2-4 kG. The spindle arm was locked at an angle θ = 2 ° to prevent workpiece rotation. As shown by this depth profile, the finishing spot has a "D" shape, and the point where the lens surface penetrates the suspension the deepest is the peak removal area. Peak removal is 4.6 μm / min and volume removal is 0.48 mm ³ / min.

This finishing spot is dependent on the type of material. FIG. 17 shows an interferogram of spots taken with two different types of glass, namely fused silica and SK7. For a fused quartz member, this spot is lowered into the suspension to a height of 1 mm above the carrier surface, at an angle θ = 0 °, the magnetic field is turned on for 20 seconds, and the magnetic field is turned off and the member is removed from the suspension. It was obtained by lifting. Depth profile line scans are taken in directions parallel (‖) and perpendicular (⊥) to the direction of flow and are indicated below the spot. These show a peak removal rate of 2.3 μm / min for this glass. For SK7 members, teeth, spots are obtained by first turning on the magnetic field. The component mounted on the spindle then moves into the suspension up to a height of 1 mm above the plane of the carrier within a range of near-right angles of incidence. There it is held for 4 seconds and then returned. Due to its composition and physical properties, SK7 finishes faster than fused quartz. The measured peak removal rate is 9.4 μm / min. The spot shapes of such glasses are very similar. This is the characteristic of the MR process.

The second input to the FDFA in the initial surface error profile of the surface to be finished is another interferogram for the sphere, which indicates the initial deviation from the most suitable sphere. For the surface of the stadium, the input can be a surface error profile obtained with a stylus tool such as Rank Taylor Hovsen From Talyserf.

The third input is a processing target. This is a subsurface damage
It can be dc removal, shape correction, or both.

The computer code combines the removal function with the initial surface shape to derive an operating program for the spindle arm angle controller of the MRF device. This code specifies the angle and acceleration of the controller, the number of moves required between positive and negative angles, and the total estimated processing time. Finally, the code can indicate the expected shape of this processing cycle.

In the embodiment shown in FIG. 1, the operation program for the spindle arm angle controller is "virtual
Can be withdrawn using "Pivot Points". The virtual pivot point can be determined when finishing any surface. This virtual pivot point coincides with the center of the sphere containing the surface to be finished. The virtual pivot point is fixed relative to the workpiece with respect to the spherical surface. In the case of a convex work piece, this virtual pivot point is above the work piece, while in the case of a concave work piece, the virtual pivot point point is below the work piece surface. In the case of a sphere, this changes appropriately to match the local curvature of the zone to be polished.

For the spindle arm controller, the MCOP moves the spindle arm so that the virtual pivot point is in place. Here, MCOP
Is the radius of curvature of the member to be finished. For non-spheres, the bending equation must be provided as input. This virtual pivot point approach allows for mimic rotation with three degrees of freedom around any point in space. This pivot point can be constant (for spherical) or variable (for non-spherical). Without the use of simulated or virtual pivot points, several dedicated devices would be required to perform the many tasks achievable with a single device.

Example 1 Table 5 illustrates the results of a three cycle process using FDFA to illustrate dc removal, shape correction, and surface polishing. This work piece is a spherical convex fused quartz member, Op
40 mm diameter and 58 mm radius of curvature generated on ticam® SX.

The first cycle lasts 32 minutes and is uniformly 3 μm from the surface
Removed and reduced the surface roughness from 40 ° to 8 ° rsm (measured with a Zygo Maxim® 3D optical profiler without filter). The symmetric surface wave front error was maintained at 0.11 μm increase for 3 μm removed material. The second cycle reduced the shape error from 0.42 μm to 0.14 μm. This was achieved in 6 minutes when radially selective removal of material up to 0.7 μm was achieved. The third cycle removed 3 μm extra material while reducing the symmetrical shape error to 0.09 μm. The final drop has a surface roughness of 8 mm
rms did not change.

A portion of the Forbes / Dumas user interface for cycle # 2 is shown in FIG. Interferograms for initial, expected, and actual surface shape errors are shown at the top of the figure. Underneath each interferogram is a line scan of the radial section showing the symmetrical tip error compared to the best-fit sphere. Note that this shape correction cycle removed the center hole in the surface.

Example 2 Table 6 shows the results of a two-cycle finishing step using FDFA, in which removal and polishing of the surface were performed in cycle # 1, and the shape was corrected in cycle # 2. This work piece is a non-spherical convex BK7 glass member with a diameter of 4
Radius of curvature generated on Opticam® SM at 7 mm
It had a non-ball departure from 70 mm to 140 μmno.

The first cycle lasts 100 minutes and is uniformly 12μ from the surface
m and reduce the surface roughness from 9400Å to 10Årms (Zyo
go New View® 20x Mirau Optical Profiler), removing all subsurface damage. The symmetric surface wave front error was reduced from 6.42 μm to 4.40 μm. The second cycle reduced the shape error from 4.40 μm to 0.86 μm. This was achieved in 40 minutes by selectively removing 4 μm material radially. 10 表面 final surface roughness
rms.

The finishing steps of Examples 1 and 2 were performed on an apparatus having a mechanically fixed pivot point. However, the work piece of Example 2 was non-spherical, deviating from the spherical shape by 140 μm. As a result, the gap between the carrier surface and the workpiece changed during the finishing operation.
Example 2 shows that the finish spot is relatively insensitive to the height of the gap between the carrier surface and the work piece, because the finish spot does not change substantially on this non-spherical surface.

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Coledonsky, William Belarus, 220088, Minsk, Prichova Street 29, Apartment 15 (72) Inventor Prokolob, Iger Victrovic Belarus, 220037, Minsk, Mendlub Street 8, Flat # 123 (72) Inventor Gorini, Donald United States of America, New York 14618, Rochester, Palmerstone Road 31 (72) Inventor Gorodokin, Jennady Lafay Rovic Belarus, 220114, Minsk, Skorina Avenue 95F (72) ) Inventor Strafford, Tubasta David United States, New York 14620, Rochester, Univer US Pat. No. 5,577,948 (US, A) US Pat. No. 5,577,948 (JP, A) US Pat. No. 5,577,948 (JP, A) 58) Field surveyed (Int.Cl. ⁷ , DB name) B24B 31/112

Claims (33)

(57) [Claims] 1. A method for finishing a surface of a workpiece using a magnetorheological fluid, comprising: (a) defining a narrowing gap between a portion of the surface of the workpiece and a continuous carrier surface. Positioning a work piece near the carrier surface, the carrier surface extending along a rim of a vertically oriented wheel; and (b) applying a magnetic field substantially at the location of the gap. (C) disposing a magnetorheological fluid from a source of magnetorheological fluid on the carrier surface; and (d) temporarily engaging a material on a portion of the surface of the workpiece to remove the material. Rotating the wheel so that the field-hardened magnetorheological fluid flows through the narrowing gap that defines the machining zone that forms the final finishing tool (E) moving one of the work piece and the processing zone with respect to the other so that a different portion of the work piece surface is predetermined with respect to the work zone. (F) collecting the magnetorheological fluid flowing through the gap from the carrier surface, exposing the exposed portion of the work piece to a predetermined extent; C.) Returning the collected magnetorheological fluid to said source of magnetorheological fluid. 2. The method according to claim 1, wherein the wheel is rotatable about a horizontally oriented axis, and rotating the wheel includes rotating the wheel about the axis. The method described in. 3. The method of claim 1, wherein the step of directing the flow of the magnetorheological fluid includes ejecting the magnetorheological fluid from a nozzle. 4. The method of claim 1, wherein the wheel has a trough-shaped outer surface such that a diameter of a central portion of the outer surface is smaller than a diameter of an edge. 5. The method according to claim 3, wherein the nozzle jets a magnetorheological fluid toward the carrier surface in a direction substantially in contact with the carrier surface and in a moving direction of the carrier surface. 6. The method of claim 1, further comprising the step of providing a predetermined geometry to the flow of the magnetorheological fluid to change the shape of the processing zone after step (c). 7. The method of claim 1, wherein applying the magnetic field includes maximizing a perimeter field present at the location of the processing zone. 8. The method of claim 1 wherein a collector for collecting magnetorheological fluid flowing through the gap is magnetically isolated to reduce the strength of the magnetic field at the collector. 9. The method of claim 1, wherein step (e) includes rotating a processing edge with respect to the processing zone. 10. The workpiece is mounted on a pivoting workpiece holder, and step (e) includes pivoting the workpiece holder to sweep the surface of the workpiece through the machining zone. Item 1. The method according to Item 1. 11. The method of claim 1, wherein step (e) includes moving the workpiece in a plane. 12. The step of moving the processing side in a plane includes moving the processing piece in a direction substantially perpendicular to the direction of movement of the magnetorheological fluid.
The method described in. 13. The method of claim 1, further comprising the step of monitoring the viscosity of the magnetorheological fluid collected in step (f).
The method described in. 14. The step of monitoring the viscosity of the magnetorheological fluid includes causing the magnetorheological fluid to flow at a substantially constant speed through a tube and measuring a pressure drop along the tube. 14. The method of claim 13, comprising: comparing the pressure drop with a predetermined value. 15. The step of moving the work piece in a plane includes moving the work piece in a direction substantially perpendicular to the direction of movement of the magnetorheological fluid.
The method described in. 16. The method of claim 13, further comprising the step of adjusting the viscosity of the magnetorheological fluid to a predetermined level when a change from a predetermined viscosity level is detected in the step of monitoring the viscosity of the magnetorheological fluid. Method. 17. The method of claim 16, wherein adjusting the viscosity of the magnetorheological fluid includes adding a carrier fluid to the magnetorheological fluid. 18. Monitoring the pH level of the magnetorheological fluid collected in step (f) and measuring it at a predetermined pH
2. The method of claim 1, further comprising maintaining at a level. 19. The method of claim 1, further comprising the step of rehomogenizing the magnetically hardened portion of the magnetorheological fluid collected in step (f). 20. The method of claim 1, further comprising the step of limiting exposure of the magnetorheological fluid collected in step (f) to outside air before repositioning the magnetorheological fluid on the carrier surface. The described method. 21. The method of claim 21, wherein the predetermined volume of the magnet rheological fluid is
19. The method according to claim 18, wherein the pH level is between 9 and 11. 22. An apparatus for finishing a surface of a workpiece using a magnetorheological fluid, comprising: a movable carrier surface with a vertically oriented outer edge of a wheel; and a magnetrheology from a source of magnetorheological fluid. A nozzle for locating a fluid flow on the carrier surface; a workpiece holder for holding the workpiece and positioning a portion of the workpiece surface near the carrier surface to form a narrowing gap between the two. Wherein the carrier surface and the work piece surface are movable relative to each other such that the magnetorheological fluid flows through the narrowing gap, and further applying a magnetic field to the gap, the gap Hardens the magnet rheological fluid flowing therethrough, engages some material on the surface of the workpiece, and A magnet forming a temporary finishing tool for removing the workpiece or the working zone with respect to the other, exposing different parts of the surface of the workpiece to said working zone for a predetermined time; Moving means for selectively finishing said portion of the surface of the workpiece to a predetermined extent; a collector for collecting magnet rheological fluid flowing from said carrier surface through said gap; and a magnet rheological fluid to said magnet rheological fluid source. An apparatus having recirculation means for returning. 23. The apparatus according to claim 22, wherein said wheel is formed of a non-magnetic material. 24. The apparatus of claim 22, wherein said carrier surface is convexly curved across said edge width. 25. The apparatus of claim 22, further comprising viscosity monitoring means for monitoring the viscosity of the magnetorheological fluid collected by said collector. 26. The viscosity monitoring means includes a tube for conveying a magnetorheological fluid at a substantially constant speed, a pressure sensor for measuring a pressure drop along the tube, and a pressure sensor for measuring a pressure drop along the tube. 26. The device according to claim 25, further comprising: comparing means for comparing with. 27. The apparatus of claim 22, further comprising cooling means for cooling the magnetorheological fluid collected by said collector. 28. The apparatus of claim 22, further comprising a mixer that rehomogenizes a portion of the magnetorheological fluid collected by the collector and solidified by the magnetic field. 29. The apparatus according to claim 22, wherein said collector comprises a scraper for raking the magnetorheological fluid from the carrier wheel. 30. The apparatus of claim 22, wherein said magnet has a pole piece and said collector is located further from said pole piece than said nozzle. 31. A rotating work piece holder to which a work piece can be mounted, said work piece holder rotating about an axis for sweeping a surface of the work piece through said working zone. 23. The device according to 22. 32. The apparatus according to claim 22, further comprising moving means for moving the work piece in a plane. 33. The magnet according to claim 22, wherein said magnet is mounted on a support base, said support base being rotatable with respect to a workpiece.
An apparatus according to claim 1.